Ten days ago the Ministry of Defence website ran an article on how the men of 1st Battalion The Royal Regiment of Scotland, in Afghanistan at the start of a six-month tour, were “hard at work building relationships with their Afghan National Army counterparts in Helmand”. The piece was illustrated with a photo of the men playing football with the Afghans at a checkpoint in Nad-e Ali.

Yesterday we learned of the death of yet another British soldier, Captain Walter Barrie, in yet another “insider attack”. Early reports suggest that the assailant, a member of the Afghan National Army, opened fire during a Remembrance Day game of football.

His battalion is part of the Brigade Advisory Group to 4th Mechanised Brigade, which makes up the bulk of UK forces in Afghanistan. It’s a tough job requiring small teams of infantry soldiers, sometimes just pairs, to work as closely as possible with the Afghan National Army, exemplifying the shona ba shona (shoulder to shoulder) slogan of the partnership.

Speaking recently to the men of 3rd Battalion The Rifles, who have just returned from six months doing exactly the same job, I was struck by how little their tour seemed to have been affected by the dramatic rise in “green-on-blue” attacks, one of the most significant and unwelcome developments in the past year. Either optimistically or stoically, those who had worked most closely with the Afghan National Army seemed least concerned by the trend. They pointed out that each incident was different and, in the context of the huge expansion of the Afghan National Security Forces, relatively isolated.

There is a tendency to ascribe too wide a significance to individual incidents in Afghanistan. But this latest “green on blue” attack comes loaded with worrying symbolism, even beyond the perversion of both Remembrance Day and the pan-cultural camaraderie of a football match. In contrast with recent fatal attacks in more remote areas, Shawqat is, by Helmand standards, a large and relatively secure operating base where the stress of combat is less acute and Afghan and ISAF troops more comfortably delineated.

Of similar concern is that it seems that the assailant was not a stranger in possession of a uniform, nor even a rogue member of the less rigorously vetted and less operationally stable police and various auxiliary police units. Instead he had come through the formal recruitment and training processes of the Afghan National Army itself. As what the Armed Forces call “combat indicators”, these are all particularly troubling.

I first visited Shawqat in 2009, not long after three Grenadier Guardsmen and two Royal Military Policemen were killed in what was effectively the first major “green on blue” incident. The Afghan soldiers of 3/215 Maiwand Corps (a unit some would say is inauspiciously named after the routing of the British by the Afghans in 1880) were universal in their condemnation of the rogue policeman who had carried out the attack. Then, the incident was startling for its singularity.

Now, the pattern is becoming ominously familiar. The incoming advisers recognise the importance of building relationships: they will need to do so quickly.

Patrick Hennessey’s new book is Kandak: Fighting with Afghans (Allen Lane)